Question

若x^2+2xy+y^2-a(x+y)+25是完全平方公式,则a的值为

要写解析过程

Answer 1

x^2+2xy+y^2-a(x+y)+25 = (x+y)^2 - a(x+y) + 5^2
-a = +-(2*5)
即a = 10 或 -10

追问

第二行不懂，怎么解的

追答

(m+n)^2 = m^2 + 2*m*n + n^2
(m-n)^2 = m^2 - 2*m*n + n^2
把m看做(x+y),n看做5